1) Field of the Invention
The invention relates to an integrated optical circuit using thin film optical waveguides for modulating or switching a light signal by an electro-optic effect and, more particularly, to a X- or almost Y-cut, near Z propagating digital optical switch.
2) Description of Related Art
The growing utilization of optical fibers in communications, sensors and other applications has made optical switching a subject of great interest. Of particular interest is the switching of signals in optical form without electro-optical transformations. Attempts have been made in a number of directions, such as by using mechanical switches and integrated optical semiconductor switches made of semiconductor material such as GaAs and InP.
Many of these switches use an electro-optical effect for the switching function. A quadratic or Kerr electro-optical effect is present in all substances and refers to a change in refractive index .DELTA.n proportional to a square of the applied electric field E. Much larger index changes can be realized in single crystals that exhibit a linear or Pockel's electro-optic effect. In this case the change of refractive index .DELTA.n is directly proportional to the applied electric field E. The effect is present only in noncentrosymmetric single crystals, and the induced index change depends upon the orientation of the electric field E and the polarization of the light beam. Well known linear electro-optic materials include potassium dihydrogen phosphate (KDP) and its deuterated isomorph (DKDP or KD*P), lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3), and semi-conductors such as gallium arsenide (GaAs) and cadmium telluride (CdTe).
To date, one of the most mature technologies for fabrication of integrated optics circuits (IOC's) is Ti:LiNbO.sub.3, which involves a ribbon guide formed by diffusing titanium into lithium niobate. The lithium niobate guide is formed in a monocrystalline wafer upon which longitudinal electrodes are placed adjacent the ribbon guide. The electrodes modify the local value of the refractive index when a potential difference is applied.
In this technology, relatively low loss channels with good spot-size matching to single mode fibers can be conveniently fabricated. However, there is one important problem; the LiNbO.sub.3 components are inherently polarization dependent, i.e., they require excitation of a specific linear polarization ("ordinary" transverse electric mode TE or "extraordinary" transverse magnetic mode TM, for a Z-cut example), dependent on crystal orientation, whereas the state of polarization (SOP) at the output of a single mode fiber will exhibit a random behavior. This problem is usually solved by using techniques adapted to accommodate the inherent polarization dependency of the LiNbO.sub.3 components, which serves to complicate the systems.
There are a number of concepts for polarization independent switches and modulators. See, e.g., M. Kondo et al, "Low Drive Voltage and Low Loss Polarization Independent LiNbO.sub.3 Optical Waveguide Switches," Electron. Lett., Vol 23, (1987), pp 1167-1169; R. C. Alferness, "Polarization Independent Optical Directional Coupler Switch Using Weighted Coupling," Appl. Phys. Lett., Vol 35, (1979), pp 748-750; O. G. Ramer et al., "Polarization Independent optical Switch with Multiple Sections of .DELTA..beta. Reversal and a Gaussian Taper Function, " IEEE Journ. Quantum Electron. Vol QE-18 (1982), pp 1772-1779; L. McCaughan, "Low Loss Polarization Independent Electrooptical Switching at .lambda.=1.3 .mu.m," IEEE Journ. Lightwave Techn., Vol LT-2, (1984), pp 51-55; Y. Bourbon et al., "Polarization-Independent Modulator with Ti:LiNbO.sub.3 Strip Waveguides," Electron. Lett. Vol 20, (1984), pp 496-497; N. Tsukada et al., "Polarization-Insensitive Integrated-Optical Switches: A New Approach," IEEE Journ. Quantum Electron., Vol QE-17, (1981), pp 959-964; J. E. Watson, "A Low-Voltage Polarization Independent Guided Wave Direction-Coupler switch in Lithium Niobate," SPIE Vol 835, Integrated Optical Circuit Engineering V, (1987), pp 132-135; J. E. Watson et al., "A polarization Independent 1.times.16 Guided-Wave Optical Switch Integrated on Lithium Niobate," Journ. Lightwave Techn., Vol LT-4, (1986), pp 1717-1721; W. K. Burns et al., "Interferometric Waveguide Modulator with Polarization-Independent Operation," Applied Physics Letters, Vol 3, (1978), p 944; P. Granestrand et al., "Polarization Independent Optical Switches," Fourth European Conference on Integrated Optics (ECIO '87) pp 36-39; P. Granestrand et al., "Polarization Independent Switch and Polarization Splitter Employing .DELTA..beta. and .DELTA..beta. Modulation, " Electron. Lett. 1988, 1142-1143; J. L. Nightingale et el, "Low-Voltage Polarization Independent Optical Switch in Ti-indiffused Lithium Niobate," Techn. Digest of Integrated and Guided Wave Optics Conf. (IGWO '89), paper MAA3, pp 10-13; K. Takizawa et al., "Polarization-Independent and Optical Damage-insensitive LiNbO.sub.3 Interferometric Waveguide Modulator," Japanese Journal of Applied Physics, Vol 27, (1988), pp L696-L698; Y. Silberberg et al., "Digital Optical Switch," Techn. Digest OFC 1988, paper THA3; H. F. Taylor, "Polarization Independent Guided wave Optical Modulators and Switches," IEEE Journ. Lightwave Techn., Vol LT 3 (1985), pp 1277-1280; T. Pohlmann et al., "Polarization independent switches on LiNbO.sub.3," Proceedings of the Topical Meeting on Integrated Photonics Research, Hilton Head, SC, 1990, pp 38-39. The first experimental results on polarization independent switches were reported by Alferness in 1979.
An approach to making polarization independent switches is to utilize crystal orientations where the conditions are similar for the two polarizations. This means that the electro-optically induced perturbations are equal and that the TE and TM modes have approximately the same coupling lengths. The "isotropic" orientations with the Z-axis in the propagation direction are examples of such orientations. Here both polarizations see the ordinary refractive index and therefore the index perturbations due to titanium (Ti) concentration are equal for the two polarizations. This means that the coupling lengths are approximately equal. The electro-optically induced index perturbations for the two polarizations are caused by the electro-optic r-coefficients r.sub.12 and r.sub.22 (contracted index notation). They have equal magnitudes but opposite signs and the index 2 which is common in r.sub.12 and r.sub.22 implies that these perturbations correspond to external electrical fields along the Y-axis. One complicating factor in this context is the fact that the two polarizations are almost synchronous and that there is one electro-optic coefficient (of the same magnitude as r.sub.12 and r.sub.22) which performs a coupling between the two polarizations. This r-coefficient is r.sub.61, indexed 1, 2, 1 in non-contracted notation. The index 6 (1,2 non-contracted) corresponds to coupling between electrical fields along the X and Y axis directions (the notation format is explained below). The coupling is induced by an external electrical field along the X axis as indicated by the second listed index.
To get good performance in a switch in this orientation, this TE-TM conversion must be avoided. However, since the unwanted (TE.fwdarw.TM) and the wanted (.DELTA.n.sub.TM, .DELTA.n.sub.TE) perturbations corresponds to different components of the external field, it is possible to avoid this coupling by proper design of the component.
One advantage with Z-propagating concepts is that the same index (ordinary) is seen by the two different polarizations. Therefore, there will be no bandwidth degradation due to pulse broadening which occurs when different indices are seen by the two polarizations.
This index difference (which implies different velocities for the portions of a pulse in the TE and TM polarizations, respectively) will appear when crystal orientations with the Z axis perpendicular to the propagation direction is used and will limit the permitted bitrate to approximately 10 Gbit/s per channel for a 10 cm long chip in these orientations. However, if the information is stacked by some other principle, such as wavelength division multiplexing or by coherent techniques, information band-widths in the THz-range can be switched even with switches in these orientations.
Another advantage with the Z-propagating concepts is that both polarizations will have approximately the same transfer function (in general, for other concepts the polarization independency means that it is possible to put the switch in two polarization independent switch states but without independency in "intermediate" points) This is most important, e.g., when linear (small signal) modulation applications are considered.
All of the switch types mentioned above are so called interferometric switches; they are based on constructive and destructive interference of modes, therefore they all have oscillating transfer functions. There is, however, another possibility, and that is to use devices which are based on mode sorting instead of interferometry. See, W. K. Burns et al., "Mode Conversion in Planar Dielectric separating Waveguides", IEEE Journ. of Quantum Electron., Vol QF-11 (1975), pp 32-35, and Y. Silberberg et al., "Digital optical Switch," Tech. Digest OFC, 1988, paper THA3.
FIG. 1 shows a mode sorting switch 100. It is a 2.times.2 digital optical switch and consists of an asymmetrical Y-branch waveguide 101 at one side and a symmetrical Y-branch waveguide 102 at the other. The latter branch 102 can be made asymmetric by applying an electrical field via the electrodes 103 and 104. An asymmetric Y-branch 101 performs mode sorting, given that the transformation is "adiabatic" (sufficiently slow). The mode sorting here means that the channel mode in the input waveguide with highest effective index gradually transforms along the branch to the first order local normal mode (fundamental mode) of the two mode region where the channels are near and influence each other (there is no power transfer between the local normal modes; at large separation the first order mode has the shape of the channel mode). In the same way, the mode in the other channel transforms to the second order mode (i.e., first higher order mode). Thus, the signal in the wide channel transforms to the first order mode in the middle region and the signal in the narrower channel transforms to the second order mode of the middle region. If the other half 102 of the switch 100 is also asymmetric (i.e., an odd index perturbation is induced) by applying a voltage to the electrodes 103 and 104 in a similar way, the first order mode in the middle region (corresponding to the wide input channel) transforms to the output channel with highest index and conversely the second order mode transforms to the output channel with lowest index.
Since the output Y-branch asymmetry can be electro-optically altered the device works as a 2.times.2 switch provided that the index perturbation is large enough and the transformation is adiabatic. If zero voltage is applied to the electrodes, a 3 dB splitting will occur for both signals.
In FIG. 2 a transfer function for a X-cut digital optical switch for TE (solid line) and TM (dashed line) is shown (note the difference). As can be seen, the transfer function does not have the oscillatory behavior of interferometric switches and the switch will operate independent of polarization assuming that the magnitude of the drive voltage is high enough.
A significant advantage of the digital optical switch is its superb stability performance. The instabilities caused by DC drift and temperature variations appear as variations of the "effective" applied voltage, and the digital response with its small transfer function slope attenuates the induced switch state perturbation if an operating point with sufficiently high voltage magnitude is chosen.
Another advantage of the digital optical switch is the power splitting achieved at zero voltage. This is especially important when broadcasting operation is requested, e.g., in some switch matrix applications.
FIG. 3 shows a switch matrix structure in which the digital switch is a very attractive choice of switch element. See, R. A. Spanke, "Architectures for Large Non-blocking Optical Space Switches," IEEE Journ. of Quantum Electron., Vol QE-22 (1986), pp 964-967. Here 1.times.2 switches are needed, which means that the switch described above can be simplified to the 1.times.2 digital optical switch structure of FIG. 4 (X-cut example).
In the switch shown in FIG. 4, the signal in the incoming single mode channel 131 excites the first order mode of the two mode region 132, and this mode is transformed (as described above) to the output with highest effective index by application of an appropriate electric field via three electrodes 133, 134, 135 (i.e., 133 and 135 being grounded and 134 being at positive or negative potential with sufficient magnitude, or 134 being grounded and 133 and 135 being at positive or negative potential with sufficient magnitude). The structure of FIG. 3 has good cross-talk properties because a signal must go wrong in two switches before it reaches an unwanted output if the matrix is properly set (this is not true in passive splitting-active combining operation as further explained infra). See, the Spanke article.
The good cross-talk performance of the structure relaxes the cross-talk requirement on the individual switch elements, which is advantageous because it is probably more difficult to achieve extremely low cross-talk with the digital optical switch than with an electronically adjustable directional coupler such as shown in the Granestrand et al. article.
Another important feature of a switch matrix due to FIG. 3 with digital optical switches as switch elements is the possibility to conveniently implement broadcast functions where the signal from one input is distributed to several outputs. In this case some (or all) of the switches in the first half of the matrix is set to 3 dB power splitting which in the case of the digital optical switch is reached at zero voltage.
Each of the couplers or switches described above suffer drawbacks such as complex fabrication, elaborate implementation and/or control, limited bitrate capacity and undue sensitivity to the environment.